The advent of wireless forms of communication necessitated the need for antennas. Antennas are required by communications and radar systems, and depending upon the specific application, antennas can be required for both transmitting and receiving signals. Early stages of wireless communications consisted of transmitting and receiving signals at frequencies below 1 MHz which resulted in signal wavelengths greater than 0.3 km. A problem with such relatively large wave-lengths is that if the size of the antenna is not at least equal to the wavelength, then the antenna is not capable of directional transmission or reception. In more modern forms of wireless communications, such as with communications satellites, the frequency range of transmitted signals has shifted to the microwave spectrum where signal wavelengths are in the 1.0 cm to 30.0 cm range. Therefore, it is practical for antennas to have sizes much greater than the signal wavelength and achieve highly directional radiation beams.
Many antennas have requirements for high directivity, high angular resolution, and the ability to electronically scan or be reconfigured. These functions are typically accomplished using a phased array antenna. A phased array antenna includes a collection of radiating elements closely arranged in a predetermined pattern and energized to produce beams in specific directions. When elements are combined in an array, constructive radiation interference results in a main beam of concentrated radiation, while destructive radiation interference outside the main beam reduces stray radiation. To produce desired radiation patterns, each individual radiating element is energized with the proper phase and amplitude relative to the other elements in the array.
In satellite communications systems, signals are typically beamed between satellites and fixed coverage region(s) on the Earth. With the expanding applications of satellites for many different aspects of communications, market requirements are continuously changing. Accordingly, a satellite must be capable of adapting to changes in the location of the requests for service. Thus, antennas provided on satellites must be capable of reconfigurable coverages.
A reconfigurable multiple beam phased array antenna is an ideal solution to the ever changing beam coverage requirements. Beam coverage can be in the form of a number of spot beams and regional beams located over specific regions. Spot beams cover discrete and separate areas such as cities. Regional beams cover larger areas such as countries. Regional beams are generated by combining a plurality of spot beams. Spot beams are generated by energizing the radiating elements with selected amplitudes and phases. A reconfigurable multiple beam phased array antenna should be capable of reconfiguring the location of the beams, the size of the beams, and the power radiated in each beam.
A problem with prior art reconfigurable multiple beam phased array antennas is that they deal with uniform sized beams and employ a large number of phase shifters which are used to steer the beams. The number of phase shifters is typically the number of elements multiplied by the number of beams. Further, the prior art reconfigurable multiple beam phased array antennas have limited bandwidth due to frequency scanning of the beams. The limited bandwidth causes the antenna gain and the co-channel interference (C/I) to degrade.
What is needed is a reconfigurable multiple beam phased array antenna in which beam locations are independent of frequency and, as a result, wider bandwidths can be achieved. To satisfy this need, the present invention provides a reconfigurable multiple beam phased array antenna employing a two dimensional stack of Rotman lenses in a low level beam forming network.
A Rotman lense is an inherently broadband beam former for linear, planar, and conformal microwave phased array antennas. The Rotman lense has been described in the technical literature including the paper "Wide-Angle Microwave Lens for Line Source Applications" by W. Rotman and R. F. Turner, published in the IEEE Transactions on Antennas and Propagation, Vol. AP-11, pp. 623-632 (1963). Other descriptions with additional information on the Rotman lense and its applications include the paper "Lens-Fed Multiple-Beam Arrays" by Donald Archer published in the Microwave Journal, Vol. 18, October 1975, pp. 37-42; and a paper entitled "Microstrip and Triplate Rotman Lenses" by A. Y. Niazi, M. S. Smith and D. E. N. Davis, published by Microwave Exhibitions and Publishers, Sevenoaks, Kent, England, in the Conference Proceedings--Military Microwaves "80".